I'm stuck on the Vahtacen's Secret quest in the Mages Guild questline. I know the order of the spells that I need to cast on the pillar and I start going in that order by casting fire, frost, and then sever magicka which then doesn't work and the pillar closes. What is going on? Somebody please help me!

try this

I already watched both parts of that video (you posted part 2 twice) and they didn't help at all. I still get the problem even when I do exactly what that guy does.

Make sure you're using Damage Magicka, and not drain magicka or absorb magicka.